Various types of hydraulic transmissions have been proposed each comprising a hydraulic variable delivery pump and a hydraulic variable displacement motor. Pumps and motors of swash plate and plunger type are well known for use as the above hydraulic variable delivery pump and variable displacement motor. In addition, there is another type of hydraulic continuously variable transmission which is known as hydromechanical continuously variable transmission (e.g., Japanese Laid-Open Patent Publication No. S62(1987)-147148). This type of transmission includes a mechanism for carrying out mechanical power transmission, which is provided parallel to a power transmission mechanism that comprises a hydraulic pump and a hydraulic motor. The above mentioned former type, which comprises only a hydraulic pump and a hydraulic motor, is generally referred to as "hydrostatic continuously variable transmission". However, here, in this document, both types, including the latter, are termed generally as "hydraulic continuously variable transmission."
Vehicles equpped with such hydraulic continuously variable transmissions are driven by the output torque of the hydraulic motor being transmitted to the wheels. This driving torque corresponds with the pressure in the hydraulic closed circuit which connects the hydraulic pump and the hydraulic motor, and the torque is generated by the oil delivered from the pump which is rotated by the prime mover (engine). Therefore, by controlling the pressure generated in the hydraulic closed circuit, it is possible to appropriately control the transmission torque between the hydraulic pump and the hydraulic motor as well as the driving torque that is transmitted from the hydraulic motor to the wheels. For example, Japanese Patent Publication No. H4(1992)-27427 discloses such a control for the pressures of two oil passages that constitute the hydraulic closed circuit. In this control, the pressure of one of the two oil passages that is higher than the other while the hydraulic pump is driven by the engine is controlled in relation to the rotational speed of the engine, and the pressure of an oil passage that becomes higher than the other when an engine brake is acted on the transmission which is driven in the opposite direction by the wheels or the vehicle in deceleration, is controlled in relation to the speed of the vehicle.
The hydraulic continuously variable transmission which is described in this patent publication comprises a fixed capacity pump and a variable displacement motor. As the capacity of the pump is constant, the relief pressure is controlled in relation to the rotational speed of the engine so that the vehicle will start smoothly.
There is another purpose for controlling the relief pressure of the hydraulic closed circuit. This control is also aimed to prevent the pressure of the oil being delivered from the hydraulic pump, which is driven by the engine, from rising momentarily abnormally high (a surge pressure) so that the hydraulic motor will not output an impact torque. In this respect, the above mentioned relief pressure control of prior art is not sufficiently effective against abnormally high pressures which are generated in various driving conditions.